Fe Al Cr Y Co ALLOY

ABSTRACT

Addition of approximately 0.5 weight percent of Co to prior art&#39;&#39;s Fe-Al-Cr-Y alloy reduces oxidation rate in air at 2,000*F to approximately one-fourth or less, and at 2,200*F to approximately two-thirds, of same alloy without Co, while producing improved tensile strength. Useful, inter alia, for automotive anti-pollution thermal exhaust reactor.

United States Patent 1191 Baxter Apr. 2, 1974 FE AL CR Y C0 ALLOY 3,170,112 2 1965 Bungardt 75/l26 H 3,298,826 H1967 Wukusick 75/124 [75] lnvemor- Walter Baxter San Jose 3,591,365 7 1971 Ohmachi 75 124 [73] Assignec: General Electric Company, New

York, NY. Primary Examiner-Hyland Bizot I Filed: y 1972 Attorney, Agent, or Firm-Allen E. Amgott 21 Appl. No.: 252,204 57 ABSTRACT Addition of approximately 0.5 weight percent of Co to [52] us. Cl. 75/124, 75/126 G p r ar Fe-Al-Cr-Y all y reduces oxidation rate in [51] Int. Cl. C22c 37/10, C22c 39/14 air at 2,000F to approximately one-fourth or less, and [58] Field of Search....;; 75/126 G, 126 H, 124 at 2, F to pp xima ly tw hird of same alloy without Co, while producing improved tensile [56]- References Cit d strength. Useful, inter alia, for automotive anti- UNITED STATES PATENTS pollution thermal exhaust reactor.

2,286,660 7/1942 Von Kantzow 75/1'24 2 Claims, 1 Drawing Figure was/v7 GAl/V (7.) cam v55 A AND a FE AL cs Y ALLOY.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to the field of oxidationresistant ferrous alloys and more particularly to the class of these comprising aluminum and chromium.

2. Description of the Prior Art The addition of chromium to iron, with or without nickel, to produce so-called stainless alloys is very old, but this stainless quality ordinarily appears only at relatively moderate temperatures, while the temperatures of interest in the present application are primarily of the order of 2,000F, but may extend to 2,200F. Pure chromium is subject to attack at high temperatures by both oxygen and nitrogen; the addition of 0.2 to 2.5

' percent (by weight, as are all the percentages herein given) of yttrium is claimed to protect chromium to above 1,700F. Iron plus 35 to 50 percent chromium is brittle and hot short; but the addition of 0.5 to 3 percent yttrium and 0.5 to 1 percent palladium produces an oxidation-resistant alloy which may be rolled first hot and then cold to sheet. Similarly, iron plus 25 to 35 percent chromium, 0.5 to percent yttrium and 0.1 to l'percent is oxidation resistant, as is iron plus to 95 percent chromium and 0.5 to 5 percent yttrium. Iron plus 20 to 95 percent chromium, 0.5 to 4 percent aluminum and 0.5 to 3 percent yttrium is oxidation resistant and cold rolls readily; with 5 percent aluminum it is brittle and is hot short. Iron plus 20 .to 35 percent chromium, 0.5 to 1.5 percent yttrium and 0.005 to 0.015 percent calcium is reported to be superior in oxidation resistance to above 2,000F and to have good workability. For use in nuclear reactors an alloy is desired which will not become brittle by hardening at high BRIEF DESCRIPTION OF THE DRAWING The single FIGURE represents the oxidation rate at two different temperatures, and the tensile strength, of alloys of interest to this application having varying proportions of cobalt.

DESCRIPTION OF THE PREFERRED EMBODIMENTS tance and in tensile strength. The single FIGURE shows, against abscissas of weightpercent of cobalt in the basic prior art alloy described above, the weight gain, in percent, of 0.030 inch thick strip when heated temperatures; an alloy of iron plus 0 to 15 percent' chromium. 0.5 to 12 percent aluminum, and 0.1 to 3.0 percent yttrium is markedly free from thisas compared with 25 percent chromium, 4 percent aluminum, and l percentlyttriumflhe preferred alloy is of iron plus 15 percent chromium, 4 percent aluminum, and 1 percent yttrium. i Y r i The uses for high-temperature resistant alloys are so numerous that it is not possible to select any one as superior to another for all applications; economic considerations, including the different costs of fabricating differently workable alloys, and the economic value of greater life in a more expensive part as compared with shorter life in a less expensive part, determine ultimate superiority for a given application.

SUMMARY OF THE INVENTION My invention is an alloy in which cobalt is added to an 'alloy of iron, chromium, aluminum, and yttrium, with a resulting improvement in resistance to corrosion at high-temperature and an improvement in tensile strength. While, as has been indicated, there are a variety of uses for high-temperature resistant alloys whose major constituents are widely and economically available, a particular present need which the alloy of my invention is adapted to fill is for the structure of exhaust reactors for internal combustion engines for which current interest in reducing atmospheric pollution foretells a substantial demand.

in air at 2,200F. for 100 hours (upper solid curve, A) and at 2,000F. for 100 hours (lower solid curve, B) as indicated by the left axis of ordinates; and the tensile strength of these alloys at 1,800F. in air in kilopounds per square inch (C and right axis of ordinates). Data points are indicated.

.The preferred manner of making these alloys is melting in vacuo with induction heating in charges of about 10 to 30 pounds, which produce an ingot about three and three-quarters inches in diameter and five inches long. This is hot extruded at 1,800F. as 1 inch round bar. Thebar is then forged flat, and hot rolled at 1,800F. to about 0.060 inches thickness. This strip is then cold rolled with intermittent air annealing at '1 ,800F. to 0.030 inch thick sheet. Photomicrographs of rnetallographic sections of samples of these alloys appear to indicate that it is preferable, in a composition which permits doing so, to add the yttrium and cobalt in the form of a master alloy of two parts yttrium and one part cobalt. This practice is believed to stabilize the oxidation-resistant oxide film formed by aluminum and yttrium, and to inhibit oxide stringer penetration through grain boundaries.

It will be observed that the weight gain (which is a measure of the oxidation rate) fall to a minimum for about 0.5 percent cobalt content,.and then rises again. i

- may be seen from the lower solid curve, for 2,000F.

for hours, an addition of 2.0 percent of cobalt raises the weight gain (and hence the oxidation rate) to about what is obtained without cobalt addition that is, for the original prior art alloy. On the other hand, it may be seen from curves A and B that some improvement, although not an optimum, may be expected from 0.25 percent cobalt. These limits represent reasonable bounds of the range of my invention which is productive of obviously useful results. Within the range, 0.4 to 1.0 percent cobalt is to be preferred. As the skilled metallurgist is aware, conventional art methods for making alloys do not produce exact compositions, and I have found by analysis that alloys in this class showed chromium from 15.0 to 15.4 percent for a nominal 15.0 percent content, and aluminum from 4.4 to 4.9 percent for a target composition of 4.0 percent; references to 3 4 the basic alloy must be understood to include such varicent chromium, 4 percent aluminum, 1 percent ytations as normal in the art. trium, cobalt from 0.25 to 1.5 percent and the remain- Numerical data represented by the curves of the der iron. drawing are: The alloy claimed in claim 1 in which the cobalt 5 Percent Cobalt Tensile Strength Percent Weight Gain in in l54l Alloy l800F. in P.S.l. 100 Hours Exposure to Atmosphere at Temperature I claim: content is from 0.4 to 1.0 percent. 1. An alloy consisting essentially by weight of 15 per- 

2. The alloy claimed in claim 1 in which the cobalt content is from 0.4 to 1.0 percent. 